Photoconductor and process for the preparation thereof

ABSTRACT

WHEREIN X1 and X2 are selected from the group consisting of hydrogen, alkyl or alkoxy with 1 to 4 carbon atoms each, or halogen, X3 and X4 are selected from the group consisting of alkyl or alkoxy with 1 to 4 carbon atoms each, or halogen, and N IS AN INTEGER BETWEEN ABOUT 6 AND 10. SAID PRODUCT BEING SOLUBLE IN TETRAHYDROFURAN AT ROOM TEMPERATURE.   This invention relates to a photoconductive polymeric condensation product of formaldehyde or para-formaldehyde and at least one multi-nuclear, carbocyclic aromatic compound, composed of recurrent, non cross-linked units of at least one of the following formulae

United States Patent [191 Liihr et al.

[451 May 6,1975

[ PHOTOCONDUCTOR AND PROCESS FOR THE PREPARATION THEREOF [75] Inventors: Bernd L'dhr, Wiesbaden; Heinz Hermann, Wiesbaden-Biebrich, both of Germany [73] Assignee: Hoechst Aktiengesellschaft [22] Filed: Apr. 19, 1973 [2]] Appl. No.: 352,498

Related U.S. Application Data [62] Division of Ser. No. 274,725, July 24, I972.

B0] Foreign Application Priority Data July 26, 1971 Germany 2137288 June 10, 1972 Germany 2228446 [52] U.S. Cl. 96/15; 252/501; 96/1.6; 260/67 A [51] Int. Cl G03g 7/00 [58] Field of Search................. 96/15, 1 R, 1.8, 1.6; 252/501, 52; 260/67 A [56] References Cited UNITED STATES PATENTS 3,705,031 12/1972 Kinjo 96/1.5 3,740,218 6/1973 Contois... 96/15 3,770,428 11/1973 Watarai 96/L5 Primary Examiner-Norman G. Torchin Assistant Examiner-.lohn L. Goodrow Attorney, Agent, or Firm-James E. Bryan ABSTRACT This invention relates to a photoconductive polymeric condensation product of formaldehyde or paraformaldehyde and at least one multi-nuclear, carbocyclic aromatic compound, composed of recurrent, non cross-linked units of at least one of the following formulae wherein X, and X are selected from the group consisting of hydrogen, alkyl or alkoxy with l to 4 carbon atoms each, or halogen,

X; and X, are selected from the group consisting of alkyl or alkoxy with l to 4 carbon atoms each, or halogen, and

n is an integer between about 6 and 10.

said product being soluble in tetrahydrofuran at room temperature.

4 Claims, 2 Drawing Figures PHOTOCONDUCTOR AND PROCESS FOR THE PREPARATION THEREOF This is a division of application Ser. No. 274,725, filed July 24, l972.

The present invention relates to a photoconductor consisting of a polymeric condensation product of formaldehyde and at least one multi-nuclear carbocyclic aromatic compound which may be substituted, which photoconductor is especially suitable for electrophotographic purposes and is preferably employed in admixture with activators and, if desired, with binders and other additives. Further, the invention relates to a process for the preparation of such photoconductor.

The use of low molecular weight polymeric condensation products of formaldehyde or para-formaldehyde and multi-nuclear carbocyclic and/or heterocyclic aromatic compounds as photoconductors is known (US. Pat. No. 3,240,597). It has been found, however, that these condensation products do not satisfy very high requirements as regards their electrophotographic and mechanical properties, such as light-sensitivity, flexibility and adhesion. This is due, on the one hand, to the aromatic compounds used, such as anthracene, naphthalene, acenaphthene, carbazole and their derivatives, whose photoconductivity is relatively low and cannot be substantially improved by condensation, and, on the other hand, is caused by the method of condensation which yields products having only a low degree of polymerization, because the monomers employed, the same as the polymeric condensation products obtained, are sparingly soluble in the solvent used. Because of the great excess of solvent used for catalyzing the condensation reaction, it is also impossible to control the reaction by maintaining a definate ratio between catalyst and monomer.

Further, high molecular weight condensation products of aldehydes with possibly substituted carbocyclic and/or heterocyelic rings are known as photoconductors (British Pat. No. 1,021,994). It has been found, however, that the photoconductive layers prepared with these compounds do not satisfy very high requirements as to light-sensitivity and chargeability. The condensation products, which are prepared in the presence of acid catalysts, especially concentrated sulfuric acid, contain, attached to the ring or to the methylol end groups formed during the reaction, sulfo groups or acid esters or easily splittable halogen groups whose presence causes substantial drawbacks for the electrophotographic properties, such as, for example, that the conductivity in the dark increases considerably and that the condensation products are no longer capable of being substantially activated. Due to the described method of preparation and the types of aromatic compounds employed, branched or three-dimensionally cross-linked condensation products are formed which do not satisfy the demands made with regard to their solubility and film-forming properties, flexibility and adhesion.

The present invention provides a photoconductor which does not have the above-described disadvantages and has a well-balanced assortment of excellent electrophotographic and mechanical properties comparable to known good photoconductors of different structure, such as polymerized heterocyclic vinyl compounds in admixture with 2,4, 7-trinitro-fluorenone (US. Pat. No. 3,484,237).

Further, the ase materials used for the preparation of the new photoconductor should be inexpensive and wherein X, and X which may be the same or different, are hydrogen, alkyl or alkoxy with l to 4 carbon atoms each, or halogen, preferably chlorine or bromine,

X and X.,, which may be the same or different, are

alkyl or alkoxy with l to 4 carbon atoms each, or halogen, preferably chlorine or bromine, and

n is an integer between about 6 and I0. and is soluble in tetrahydrofuran at room temperature.

Electrophotographic layers containing the photoconductor according to the invention posses a high degree of flexibility, good adhesion power, high luster, good film-forming properties, good chargeability, and good light-sensitivity. The polymeric condensates according to the invention are readily soluble and the solutions produced may be cast, for example, to form films of high adhesive power and elasticity.

Suitable multi-nuclear carbocyclic aromatic compounds are pyrene and its substitution products, and the derivates of perylene, which are polycondensed either alone or in admixture with formaldehyde or paraformaldehyde. Among the aromatic compounds of this type, the halogen derivatives of pyrene have proved to be particularly advantageous because of their good solubility characteristics and the relatively higher electrophotographic sensitivity of their polycondensates. The bromine derivatives are more easily available, especially because their preparation is simpler. Of the bromine derivatives, the 3-bromopyrene is preferred because it is less expensive and possesses more favorable solubility characteristics.

Good results are also obtained, particularly when the photoconductor according to the invention is used in combination with special activators, such as 2,4,7- trinitro-fluorenone, by forming a co-condensate with bianthryl. A bianthryl content of about 40 to molar per cent, calculated on the co-condensate, has proved to be advantageous.

3 Photoconductors which are highly light-sensitive within the ultraviolet range of the spectrum as shown in Curves l and ll of FIG. I. for positive and neg ative charging and which are thus particularly suitable for electrophotographic purposes. are mixed with activators (electron acceptors) such as those known from U.S. Pat. No. 3.287.] l3. in order to shift their sensitivity towards light of a longer wavelength. Activation of the photoconductors according to the invention within the visible range of the spectrum is of importance. because it enables the reproduction of colors. Layers which have been activated by means of electron acceptors display a particularly high light-sensitivity, and this high degree of light-sensitivity is achieved by adding the activators in a relatively low concentration. which is not the case in layers according to U.S. Pat. No. 3,484.237, for example. While the molar ratio between activator and photoconductor according to the present invention ranges from about 0.01 to about 0.4:l. ratios of about lzl are required in this U.S. patent to achieve maximum light-sensitivity. It has proved advantageous to employ 0.1 to about 0.4 mole of activator per mole of photoconductor. Besides their high light-sensitivity. the layers according to the invention which are used in thicknesses of about 2 to 20 ;1.. preferably of 5 to p. are distinguished by their excellent charge-ability, which in the case of negative po larity is in the range of 600 to lZOO volts. depending on the thickness of the layer. This property is of particular advantage because due to the high chargeability. high difference in voltage are achieved during image-wise exposure. such as are required for the production of high contrast images.

Besides their high chargeability, the polymeric condensation products of the invention simultaneously possess a very low conductivity in the dark. which guarantees that the high differences in voltage between light and dark areas produced during image-wise exposure are retained over a relatively long period of time.

The photoconductors according to the present invention permit a sensitization within a definite range of the spectrum. By selecting appropriate activators. the maximum sensitivity may be displaced from the short-wave to the long-wave range of the visible spectrum. Further. a sensitization over the entire range of the spectrum may be achieved by combining different activators. This possibility is less pronounced in the case of known electrophotographic layers. Suitable activators are. e.g.: mono-, di-. tri-and tetranitrofluorenone. 9- dicyanomethylene-fluorene, 9-dicyanomethylenemononitro-fluorene. 9-dicyanomethylene-dinitrofluorene, and 9-dicyano-methylene-trinitro-fluorene.

Further, binders may be added to the activated photoconductors according to the invention. Those binders are preferred which are soluble in tetrahydrofuran. e.g. polyesters containing terephthalic or isophthalic acid as acid components and various diols as alcoholic components in the chain. Normally, an improvement of the mechanical properties of the layers is achieved by the addition of a binder. The binder content may vary within wide limits without affecting the light-sensitivity of the layers. Particularly in the case of halogensubstituted polycondensates. it may amount to about 200 percent by weight, based on the weight of the photoconductor. The possibility to add to the photoconductor such large quantities of polyester, for example. brings about further advantages over known layers,

apart from the large savings in cost. As compared with the layers according to U.S. Pat. No. 3.484.237. for example. the layers according to the invention possess an extreme flexibility and adhesive power, so that they withstand all mechanical stresses which may occur, for example. in a copying machine. In addition to their improved mechanical properties. an improved chargeability and higher light-sensitivity may be achieved. Depending on their binder content, the layers described may be charged to between about L400 and 1.800 volts without disruptive discharge. The resistance to disruptive discharge results in a longer service life of the photoconductor layers. In addition to their high resistance to mechanical stress. the layers are distinguished by a particularly smooth surface which, in turn. enables an excellent toner transfer. so that copies of very high contrast are obtained and cleaning of the layer is considerably less expensive.

Further additives which may be incorporated in the described layers and are sometimes advantageous, are: plasticizers. such as dibutyl phthalate; wetting agents, such as silicone oil; dyes, such as cyanine dyes; and/or pigments, such as zinc oxide.

Further. the present invention relates to a process for the preparation of the photoconductor according to the invention from formaldehyde or para-formaldehyde and at least one multi-nuclear carbocyclic aromatic compound which may be substituted. by introduction of the components into a solvent. condensation in the presence ofa condensation catalyst at temperatures be tween and C. precipitation ofthe polycondensate, suction. purification and drying. in the process, the solvent used is dioxane and perchloric acid is used as the condensation catalyst: the polycondensate is precipitated from a mixture of tetrahydrofuran and a lower aliphatic alcohol. preferably methanol.

In a modification of the inventive process for the preparation of the photoconductor. the solvent used is glacial acetic acid and anhydrous zinc chloride is used as the condensation catalyst.

The processes according to the present invention and the above-described disadvantages and in the case of the multinuclear carbocyclic. possibly substituted, aromatic derivatives of pyrene and perylene under consideration lead to polycondensates having excellent mechanical and photoconductive properties, since no further reaction of the components with the catalysts used or incoorporation of the catalysts takes place.

The selection of dioxane as the solvent guarantees not only the solubility of compounds of a relatively high degree of condensation. but also the good catalytic ef fect of perchloric acid. When using glacial acetic acid as the solvent. a transgression of a degree of polycondensation unfavorable for the solubility in tetrahydrofuran may be avoided by the relatively limited solubility of the polycondensates formed. Moreover. this solvent is best suited for guaranteeing the optimum catalytic effect of the catalyst used. because it substantially prevents hydrolysis. i.e. the formation of hydrochloric acid. which may cause undesired reactions.

In principle. both processes may be employed for the aromatic compounds mentioned. Which process is selected depends on the solubility and the reactivity of the aromatic compounds to be condensed. In the case of an aromatic compound which is sparingly soluble. condensation in dioxane has proved to be preferable, because normally dioxanc has a better dissolving power. when the aromatic compound concerned is readily soluble in dioxane and highly reactive. the condensation reaction may be more easily controlled when glacial acetic acid is used as the solvent. Further, it is possible in both processes to control the reaction process not only by the ratio of aldehyde and aromatic compound employed, but also by the quantity of catalyst added, a fact which is of particular importance for the production of chains of a desired length.

The weight ratio between aromatic compound and condensation catalyst may vary within wide limits. During the condensation of pyrene, for example, a ratio between the aromatic compound and the zinc chloride used as a catalyst of about (5 to 8 ):l has proved advantageous, however, whereas during the condensation of bromopyrene, for example, a ratio of about (8 to ll):l is suitable. When perchloric acid is used as the catalyst in the condensation process, ratios between aromatic compound and catalyst of to ):1 for pyrene and its derivatives and of (4 to 6):] for condensates based on perylene are suitable. Similar ratios apply to cocondensates.

The preparation of the photoconductors according to the invention will be described more in detail in the following examples:

EXAMPLE I l00 g of 3-chloropyrene are introduced into 750 ml of dioxane and the mixture is maintained at a temperature of 95 C. The solution is then mixed with l to 2 ml of perchloric acid. Finally, 12.6 g of para-formaldehyde are added while stirring. The condensation is interrupted after 4 hours, the solution is allowed to cool, and is then added dropwise to 6 liters of methanol. The

polycondensate which precipitates in the form of fine flakes is filtered, washed with methanol, and dried. After drying, the polycondensate is dissolved in ISO ml of tetrahydrofuran, filtered, and reprecipitated by introducing the solution dropwise into methanol. This purification process is repeated. After drying, a colorless product is obtained which softens in the range of I to lC and which has a degree of polycondensa tion between 7 to 10 when measured by the chromatographic gel permeation method using low molecular weight polystyrene for comparison (Yield: 75 to 80% EXAMPLE 2 l2 g of dichloroperylene are dissolved at 90-95C in 300 ml of dioxane. The first 1 to 2 ml of perchloric acid and then l.2 g of para-formaldehyde are added to the solution. The condensation process lasts 3 hours. The formulation is then processed in the manner described above. Yield; 60 to 65 72. Softening range: l30-l35 C. Degree of polycondensation: 8 to ID.

EXAMPLE 3 40 g of pyrene are dissolved together with 6.5 g of paraformaldehyde at 90-l00 C in 250 ml of glacial acetic acid. After this temperature has been reached, 5.0 g of anhydrous zinc chloride are added and the formulation is then stirred for 5 hours at constant temperature. The precipitating polycondensate is drawn off by suction, washed first with water and then with methano], and finally dried. For purification, the polycondensate is dissolved in about 300 ml of tetrahydrofuran, filtered, introduced into about 2 liters of methanol, and re-precipitated. This purification process is repeated. A

colorless product is thus obtained in a yield of to The softening range is from about l55 to C. The degree of polycondensation, measured by the method cited in Example I. is about 6 to 8.

EXAMPLE 4 40 g of 3-bromopyrene are dissolved, together with 4.5 g of para-formaldehyde. in 400 ml of glacial acetic acid at 95-l00 C., 2 g of anydrous ZnCl are added and the formulation is further stirred for 3 to 4 hours. The precipitating polycondensate is drawn off by suction, washed with water and ethanol, dried, and then purified as described above. A colorless powder results which has a softening range of l30 C and a degree of polycondensation between 6 and 8. Yield: 90-95%.

EXAMPLE 5 30 g of bianthryl and 50 g of 3-bromopyrene are dissolved together with 2 ml of perchloric acid in l500 ml of dioxane at a temperature of 9095 C. Then 7.9 g of para-formaldehyde are added to the clear solution. After I hour, the condensation process is interrupted by pouring the solution into 8 liters of methanol. Processing and purification are as in Example I.

A yellowish product is obtained in a yield of 50-60%, which has a softening range of about l70 to C.

The properties of the photoconductors according to the invention will be described in the following by reference to their use in electrophotographic layers. The data measured will be compared with the corresponding data of known layers prepared according to Example 2 of U.S. Pat. No. 3,240,597 (here: Example A), Example 15 of British Pat. No. l.02l.994, (here: Example B), and Example 1 of U.S. Pat. No. 3.484,.237, (here: Example C).

EXAMPLE 6 2 g of pyrene resin with a softening range of l60-l65 C, 0.56 g of trinitrofiuorenone (molar ratio between activator and monomer unit 0.19), and L2 g of polyester (Dynapol L 203, a product of Dynamit Nobel AG., Troisdorf, Germany) are dissolved at room temperature in 25 ml of tetrahydrofuran and drawn off from the undissolved residue by suction. The solution is whirler-coated onto a rotating conductive support consisting of polyester with a vapor-deposited aluminum layer thereon.

The light-sensitive layer thus produced is dried for 3 to 4 minutes at 1 10 C in a drying cabinet to remove the solvent. A reddish-brown layer results. The layer is negatively or positively charged under a corona, then image-wise exposed, and developed in the normal manner. An image is thus produced which is rich in contrast and shows no scumming. The electrophotographic properties were determined by means of the Dyntest- 90" apparatus manufactured by ECE Gmbh, Giessen. Germany. This apparatus serves to determine charge. dark discharge, and light discharge. The half-time is used as a measure of the sensitivity. Half-time" is the time in which the voltage is reduced to half its original value at a particular exposure.

The characteristic data are as follows:

Half-Time tmsec.)

Charge (Volts) The above-mentioned comparison layers produce the following data:

Charge Half-Time (Volts) (mseo) A 480 650 l 75 (high residual voltage) 8 450 600 80 75 (considerable dark dis charge) C 600 H00 50 28 (considerable dark discharge with positive charge) EXAMPLE 7 2 g of 3-bromopyrene resin and 0.56 g of trinitrofluoreneone are dissolved, together with l g of a polyester ("Dynapol" L 206, a product of Dynamit Nobel AG. Troisdorf. Germany) in 25 ml of tetrahydrofuran at room temperature and then applied as described in Example l to polyester provided with a vapor-deposited aluminum layer.

The following values result:

Charge Half-Time (Volts) (mseel I250 I600 32 28 (very little dark discharge) EXAMPLE 8 2 g of 3-bromopyrene resin and 0.22 g of dicyanomethylene-Z.7-dinitrofluorene are dissolved, together with l g of polyester. in 25 ml of tetraltydrofuran, and the solution is filtered and cast as a film onto a rotating conductive support consisting of polyester with a vapor-deposited aluminum layer thereon. The resulting light-sensitive layer shows the following values:

Charge Half-Time (Volts) (msee) I 250 I400 l l0 EXAM P LE 9 2 g of 3-bromopyrene resin, 0.25 g of dicyanomethylene-2,4,7-trinitrofluorene, and l g of polyester are dissolved in 25 ml of tetrahydrofuran and applied as a thin film to a conductive support as described in the preceding examples. Upon examination of the film, the following values are measured:

Half-Time (msec.)

Charge (Volts) l l 00 l 300 EXAMPLE 10 2 g of 3-bromopyrene resin, (H g of tetranitropyrene. and l g of polyester are dissolved in ml of tetrahydrofuran. applied as a thin film to a conductive support as described in the preceding examples, and examined. The following values are measured:

Charge (Volts) Half-Time (msec.)

EXAMPLE ll 2 g of 3-bromopyrene resin. 0.2 g of trinitrobromopyrene. and l g of polyester are processed. in the 20 manner described aboveto yield a photoconductive layer. The following values are measured:

Charge Volts l Half-Time (msec.

l 100 I300 ll) EXAMPLE l2 2 g of 3-bromopyrene resin are activated with 0.5 g of trinitrofluoroanthene and then processed together with l g of polyester in known manner to yield a photo- 3 conductor layer.

Charge Half-Time (Volts) tmsec.)

I025 H 37.5 2K0 45 EXAMPLE l3 2 g of S-bromopyrene resin, 0.3 g of tetranitroperylene, and l g of polyester are processed in the same manner to yield a photoconductor layer.

I g of dichloroperylene resin and 0.l4 g of trinitrofluorenone are dissolved together with L5 g of polyester in 25 ml of tetrahydrofuran and used in the manner described for the preparation of a photoconductive layer.

Result:

Half-Time imsec.)

Charge (Volts) EXAMPLE l5 Charge Half-Time t Volts I tmscc.)

EX A M PLE l 6 2 g of isopropyl-pyrene resin. 0.5 g of trinitrofluorenone and 0.5 g of polyester are dissolved in 25 ml of tetrahydrofuran. The solution is filtered and then cast as a film onto a rotating conductive support. The layer is dried for 3 to 4 minutes at I I0 C in a drying cabinet to remove residual solvent. Subsequently. the layer is negatively charged by a corona. image-wise exposed. and developed in the normal manner. The toner image is transferred onto paper and fixed by heating it for 20 seconds. An image is thus obtained which is rich in contrast and free from scumming.

EXAMPLE I? The procedure described in Example I6 is repeated.

except that the isopropyl-pyrene resin is replaced by t-butyl pyrene resin or di-t-butyl-pyrene resin. good results being also obtained in this case.

EXAMPLE l8 2 g of methoxy-pyrene resin. 0.45 g of trinitrofluorenone. and l g of polyester are dissolved at room temperature in tetrahydrofuran and processed as described into a photoconductive layer on a conductive support.

EXAMPLE 19 The position of the maximum sensitivity may be displaced by using different activators. In the following schedule. the maxima are listed for 3-bromopyrene resin layers mixed with different acceptors. In all cases. the ratio between monomer unit and activator is I10. I this ratio being selected to facilitate comparison. without assuming that it effects optimum activation in each case.

No. Activator Maximum Sensitivity III Z-nitrofluorenone 400 4K0 tnm) l\' 2.7-dinitroflunrenone 4Z0 550 2.4.7-trinitrotluorenone 420 6-80 \I 2.4.5.7-tetranitrotluorenonc 420 750 \"ll 9-dicyanomethylenefluorene 420 575 \III Q-dicyanomethyIene-l 420 625 nitrofluorene IX 9-dicyanomethylene-2.7- 420 700 dinitrofluorene In addition. the distribution of the spectral sensitivities of the activated layers is indicated in the accompanying FIG. 2, the sensitivities being shown as reciprocal half-times of negatively charged layers.

The attached FIGS. 1 and 2 differ in scale. In FIG. 2, the ordinate was reduced by a factor of about 4.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof. and the invention includes all such modifications.

What is claimed is:

l. A photoconductive polymer condensation product. of formaldehyde or para-formaldehyde and at least one multi-nuclear. carbocyclic aromatic compound. composed of recurrent. non cross-linked units of at least one of the following formulae and wherein X and X; are selected from the group consisting of hydrogen. alkyl or alkoxy with l to 4 carbon atoms each. or halogen.

X and X. are selected from the group consisting of alkyl or alkoxy with l to 4 carbon atoms each. or halogen. and

n is an integer between about 6 and I0. said product being soluble in tetrahydrofuran at room temperature. and containing 0.I to 0.4 mole of activator per mole of monomer unit.

2. A product according to claim 1 containing at least one derivative of fluorenone. fluorene. pyrene. fluoranthene or perylene as an activator.

3. A product according to claim 1 which contains at least one fmnn0-. di-. triand tetra-nilro-fluoreneone.

9-dicyan0methylene-fluorene.

mononitm-fluorene.

tro-bromopyrene.

tetranilro-pyrene.

Q-dicyanomethylene- 9-dicyan0methylene-dinitrnfluurene. 9-dicyunomethylene-trinitro-fluorene. lrinitrintimweight of the photocunduclur. 

1. A FORMALDEHYDE OR PARA-FORMALDEHYDE AND AT LEAST ONE MULTINUCLEAR, CORBOCYCLIC AROMATIC COMPOUND, COMPOSED OF RECURRENT, NON CROSS-LINKED UNITS OF AT LEAST ONE OF THE FOLLOWING FORMULAE
 2. A product according to claim 1 containing at least one derivative of fluorenone, fluorene, pyrene, fluoranthene or perylene as an activator.
 3. A product according to claim 1 which contains at least one of mono-, di-, tri- and tetra-nitro-fluoreneone, 9-dicyanomethylene-fluorene, 9-dicyanomethylene-mononitro-fluorene, 9-dicyanomethylene-dinitro-fluorene, 9-dicyanomethylene-trinitro-fluorene, trinitro-bromopyrene, tetranitro-pyrene, trintiro-fluoranthene or tetranitro-perylene as an activator.
 4. A product according to claim 1 containing up to 200 percent by weight of binder, calculated on the weight of the photoconductor. 